Ag2Se as a tougher alternative to n-type Bi2Te3 thermoelectrics

For half a century, only Bi2Te3-based thermoelectrics have been commercialized for near room temperature applications including both power generation and refrigeration. Because of the strong layered structure, Bi2Te3 in particular for n-type conduction has to be texturized to utilize its high in-plane thermoelectric performance, leaving a substantial challenge in toughness. This work presents the fabrication and performance evaluation of thermoelectric modules based on n-type Ag2Se paring with commercial p-Bi2Te3. Ag2Se mechanically allows an order of magnitude larger fracture strain and thermoelectrically secures the module efficiency quite competitive to that of commercial one for both refrigeration and power generation within ± 50 K of room temperature, enabling a demonstration of a significantly tougher alternative to n-type Bi2Te3 for practical applications.

term stability of the thermometric generator by measuring its output power continuously for one month and add that data in the manuscript to show the stability of Ag2Se as well as Ag/Ag2Se & Ni/Ag2Se electrical contacts at high temperature.This additional experiment will really prove the real potential of Ag2Se over Bi2Te3.

Reviewer #3 (Remarks to the Author):
This work presents the fabrication and performance evaluation of thermoelectric modules based on n-type Ag2Se.While recent thermoelectric research on Ag2Se has primarily focused on its film performance and devices, I find this study's emphasis on Ag2Se bulk modules to be particularly intriguing.This work systematically investigated the toughness of bulk Ag2Se compared to Bi2Te3, which is interesting and informative for future researches.The results demonstrate that Ag2Se, with its superior bending and compressive strength, indeed represents a tougher alternative to n-Bi2Te3.The contact resistivity here is low.I believe this may mark the first demonstration as I didn't find any literature reporting contact resistivity in bulk Ag2Se device.The n-Ag2Se/p-Bi2Te3 module seems to be the first demonstration as well, and the performance is good.With these important results, I recommend that this work be published in Nature Communications after addressing the technical concerns 1. Paragraph 3 in part "Results and discussion": "……The transport properties can be well described by a single parabolic band (SPB) model with an acoustic scattering (Figs.S3b and  S3c).……".It is better to include the logarithmic relationship between temperature and mobility in Fig. S3 to illustrate acoustic scattering in Ag2Se.2. We generally consider n-type Bi2Te3 to be more brittle compared to p-type Bi2Te3.Why is it that here, the n-type Bi2Te3 exhibits higher bending and compressive strength?3. Geometry design is also important for maximizing the performance of thermoelectric module.Why did the authors select a leg size of 2×2×4 mm3?Please provide further explanation.4. Fig. S8 is not mentioned in the manuscript or the supplementary. 5.The internal resistances of the Ag2Se-based module and the commercial Bi2Te3 module differ.However, the optimized currents for these two modules appear to be the same, as shown in Figure 4a and Figure S8.Any explanation on this issue?
Response and revision: Thanks for the suggestion.The literature has been cited as ref.16.
3) The detailed constituent for p-Bi 2 Te 3 legs in your device as well as its electrodes should be provided for readers to repeat the results.

Response and revision:
Thanks for the suggestion.The p-type Bi 2 Te 3 legs used in this work are the same as those in the commercial modules.We desoldered the commercial modules and used their p-Bi 2 Te 3 legs to pair with our fabricated n-Ag2Se legs to form our modules.Since the detailed constituent of these p-Bi2Te3 legs are proprietary, we provided information about the source and the model number of the involved commercial module (Table S1 in the supplementary information).4) Although there are many papers on high performance Ag 2 Se thermoelectrics (having ZT above 1 near room temperature), almost no report on bulk Ag2Se thermoelectric device including the single leg [Energy Environ. Sci., 2023, 16, 1870-1906].What's the key issue that hinder the development of bulk Ag 2 Se device?In order to highlight the importance of this work, this problem needs to be discussed in the introduction as well as your solution.
Response and revision: Thanks for the suggestion.Ag2Se undergoes a phase transition from low-temperature orthorhombic phase to the high-temperature cubic phase at ~406 K 26 .The high zT was usually realized in the orthorhombic phase of Ag 2 Se and was mainly attributed to its high carrier mobility and low lattice thermal conductivity 27 .However, phase transitions are typically undesirable as they may result in volume variations, which could lead to structural damage either within the material itself or at the interface between the material and electrodes during service.This somewhat limited the researches on Ag 2 Se to focus on exploring its material properties 23, 25, 28  and fabricating film devices specifically designed to operate at room temperature 17-18, 29-31 .There are few reports on power generation and cooling performance of Ag 2 Se bulk modules.This motivates the current work to focus on exploring device properties of bulk Ag 2 Se bellowing its phase transition temperature.
5) As we know, the performance of Ag2Se is sensitive to defects like Ag interstitial and also the ratio of Ag/Se [Energy Environ. Sci., 2023, 16, 1870-1906], and the mass transport/the reaction of defects will happen between beta-Ag 2 Se and Ag electrode at high temperature sintering (673 K, above phase transition temperature) processing.One can also observe the content of Se is obviously higher in the side of silver electrode when compared with nickel electrode in Fig S7c-d, which possibly support the above-mentioned reaction.How to solve the problem of the interfacial reaction in high temperature welding for this superionic conductor Ag 2 Se at high temperature, which is important for the application of Ag2Se and needed to be discussed.

Response and revision:
Thank you for the comment.The previous SEM and EDS results of the Ag 2 Se/Ag and Ag 2 Se/Ni joints were obtained using traditional SEM equipment (Phenom Pro).Since traditional SEM is not as accurate as field emission SEM (FE-SEM) for elemental quantitative analysis, we conducted elemental analysis using FE-SEM equipment (Zeiss Sigma 300VP) to further confirm the precise content of the elements in the joints.It can be seen in updated Fig. S7c-f, Se was not detected in either the Ni electrode or the Ag electrode after hot pressing.However, the long-term measurements on two modules (Fig. 3e and Fig. S12) indeed indicate that Ag as an electrode for Ag 2 Se is not a good choice for power generation concerning long-term stability.Using Ni as electrodes for Ag 2 Se enables more stable power generation, as demonstrated by continuous measurements over 30 days (Fig. 3e).We updated Fig. S7 in the supplementary information and added a brief discussion about the long-term stability of modules using Ni or Ag as electrodes (line 93, page 2).

Response and revision:
In this work, we used low-temperature solder (In52Sn48, with a melting point of 391 K) to avoid phase transition.The soldering process was conducted at 391 K, which does not reach the phase transition temperature of Ag 2 Se (~406 K).During this process, Ag 2 Se does not undergo phase transition.In the preliminary step before soldering, the hot pressing of Ag2Se legs was indeed conducted at high temperature (573 K).We performed SEM and EDS analysis on the cross-sections of the Ag 2 Se legs that had undergone both hot pressing and soldering processes.A robust bonding without any cracks is confirmed by SEM observations taken after the hot pressing and welding processes (Fig. S8).We revised to include relevant clarification in the main text (line 74, page 2) and supplementary information (Synthesis section).
We provided CTE for n-Ag 2 Se, Ni, Ag and p-Bi 2 Te 3 involved in our modules, and added a brief discussion in the main text (line 93, page 2): Although the linear coefficient of thermal expansion (CTE) of Ag2Se differs from p-Bi 2 Te 3 at 300-373 K (Fig. S11), no obvious degradation in  max , P max , V OC and R in is observed for module 1 (Ni electrode) after continuous measurement for 30 days (Fig. 3e).However, the R in of module 2 (Ag electrode) increases obviously as the measurement time progresses (Fig. S12), leading to a notable degradation in both output power and efficiency.This indicates that using Ni as electrodes for Ag 2 Se enables superior long-term stability of the module compared to using Ag as electrodes.The inset shows the measured CTE at 300-373 K with comparison to literature results [5], [18-19] .7) Module 1 owns better performance in both power generation and cooling applications (Figure 3 and 4) than that of module 2, why?The reason for the decreased performance in module 1 with silver electrode need to be explained, the interfacial reaction?Or others.

Response and revision:
Since the contact resistivity of Ag 2 Se/Ni and Ag 2 Se/Ag are very close (Fig. S7), the reason for the better performance of module 1 is presumed to be that the Ag 2 Se in module 1 has a lower carrier concentration, which is closer to the optimal value.This is evidenced by the higher voltage output and internal resistance of module 1 (Fig. S10).We revised the manuscript to include above discussion (line 21, page 3).The manuscript titled "Ag 2 Se as a tougher alternative to n-type Bi 2 Te 3 thermoelectrics" describes the following important issues.Ag2Se is one of the best materials for near room temperature application due to the fact that its figure-of-merit is very high in temperature range of 300-380 K.I fully agree with the authors justification that it has better mechanical strength than Bi 2 Te 3 , which is very important from device fabrication point of view.My only worry is the long term stability of the n-Ag2Se/p-Bi2Te3 based thermoelectric power generation module.In Ag2Se it is well known that even at room temperature Ag+ ions can migrate and form clusters which can change the properties.The authors should test that long term stability of the thermometric generator by measuring its output power continuously for one month and add that data in the manuscript to show the stability of Ag2Se as well as Ag/Ag2Se & Ni/Ag2Se electrical contacts at high temperature.This additional experiment will really prove the real potential of Ag 2 Se over Bi 2 Te 3 .

Response and revision:
We sincerely thank the reviewer for the positive comments.In order to address the reviewer's concern regarding the long-term stability, we have measured both the power generation and cooling performance of the modules for over one month.During this process, we optimized the contact resistance of all connection points in the external circuit of the testing system, thereby improving test quality.We found that the internal resistance of all modules had been overestimated in the previous power generation tests.Therefore, we retested the power generation performance of all modules.The long-term measurement results are shown in Figs.3e,   4e, and S12.No obvious degradation in max, Pmax, VOC and Rin is observed for module 1 (Ni electrode) after continuous measurement for 30 days (Fig. 3e).However, the R in of module 2 (Ag electrode) increases obviously as the measurement time progresses (Fig. S12), leading to a notable degradation in both output power and efficiency.This indicates that using Ni as electrodes for Ag2Se enables superior long-term stability of the module compared to using Ag as electrodes.Although module 2 showed unsatisfactory stability during long-term power generation measurements, its cooling performance was found to be quite stable after 34 days of measurements (Fig. 4e).We revised to include Fig. 3e   9-10, 15-16,  20, 42, 44-46 .(e)  max , Q, P max , V OC , internal resistance R in and ΔT of module 1 during continuous measurements for 30 days at ΔT of ~85 K.

Fig. S7
Fig. S7 Contact structure and resistance.Scanning resistance (R) across Ni/Ag2Se/Ni (a) and Ag/Ag2Se/Ag (b) junctions.SEM images, EDS mapping, EDS line scanning and elemental analysis for the Ag 2 Se/Ni (c, e) and the Ag 2 Se/Ag (d, f) joints.

Fig. S8
Fig. S8 Contact structure after welding.SEM images, EDS mapping and EDS elemental analysis for the Ag2Se/Ni/solder (a) and the Ag2Se/Ag/solder (b) joints.

Fig. S11
Fig. S11 Thermal expansion measurements.Temperature dependent relative length variation (dL/L 0 ) for n-Ag2Se, Ni, Ag and p-Bi2Te3 involved in our modules.The values on the dL/L0 curves represent the linear coefficients of thermal expansion (CTE) in the specific temperature range.The inset shows the measured CTE at 300-373 K with comparison to literature results[5], [18-19] .
, Fig. 4e and above discussion in main text (line 96, page 2 and line 12, page 3), and include Fig. S12 in supplementary information.

Fig. 3
Fig. 3 Power generation performance.(a) Open-circuit voltage (VOC), (b) maximum output power (Pmax), (c) heat flow (Q) and (d) maximum conversion efficiency ( max ) as a function of different temperature gradients (T) for n-Ag 2 Se/p-Bi 2 Te 3 modules and commercial Bi 2 Te 3 one.Literature results are included in (d) for comparison9-10, 15-16,  20, 42, 44-46 .(e)  max , Q, P max , V OC , internal resistance R in and ΔT of module 1 during continuous measurements for 30 Fig. S12 Power generation duration of module 2. Maximum conversion efficiency max, heat flow Q, maximum output power P max , open-circuit voltage V OC , internal resistance R in and ΔT of n-Ag 2 Se/p-Bi 2 Te 3 module 2 during duration measurements at ΔT of ~85 K.

Fig. 4
Fig. 4 Cooling performance.(a) measured maximum cooling temperature difference (Tmax), (b) current dependent maximum cooling power (Q cmax ) and (c) coefficient of performance (COP), as well as (d) maximum COP as a function of different T for the modules 47 .(e) Cold-side temperature (T c ), hot-side temperature (T h ) and corresponding cooling temperature difference (T) of n-Ag2Se/p-Bi2Te3 module 2 at a given current I of 4.3 A during duration tests for 34 days, showing good stability.During the cooling duration tests, we run the test once per day for a continuous period of 22 hours.